1. Field of the Invention
The present invention relates to a membrane apparatus for filtering a raw liquid, and a membrane treatment apparatus employing the membrane apparatus to perform membrane treatment. Examples of the raw liquid include sludge in an aeration tank for biological treatment, sludge discharged from the aeration tank, concentrated sludge, waste water containing human excrement before being subjected to biological treatment, etc.
2. Description of the Related Art
Waste water containing organic substances, nitrogen, phosphorus, etc., which would contaminate oceans, rivers and the like is generally subjected to biological treatment to convert it into clean water and is then discharged into a river, for example.
As a means for separating the solid and liquid components of a reaction mixture resulting from biological treatment, a settling tank of a gravity settling type has conventionally been used. However, in recent years, a membrane separating technique has been used so as to reduce installation space and facilitate maintenance.
In such a membrane separating technique, it is very important to constantly produce a filtrate for a prolonged period of time. However, the amount of filtrate unavoidably decreases with passage of time. It is considered that this problem is partly caused by separated concentrated substances which deposit on the surface of a membrane and form a gel layer. The gel layer grows and hinders the passage of liquid to be filtered. The gel layer is called a concentration polarization layer, and the thickness of the concentration polarization layer increases as the concentration of contaminants in sludge increases and as the amount of filtrate increases. Accordingly, in the membrane separating technique, reduction of the thickness of a concentration polarization layer and removal of a concentration polarization layer are quite important.
Membrane apparatuses are categorized into a flat membrane type and a tubular type, etc. based on the kind of a membrane used. They are also categorized into an out-tank type in which a membrane is installed outside an aeration tank and an in-tank type in which a membrane is installed within a tank, based on the location of membranes.
FIG. 4 shows a conventional membrane treatment apparatus employing a membrane apparatus of the out-tank type using a flat membrane. In FIG. 4, numeral 50 denotes a membrane apparatus, numeral 51 denotes a circulating tank for receiving sludge fed from an unillustrated aeration tank, and numeral 52 denotes a circulating pump. Numeral 53 denotes a frame which can be disassembled after unillustrated packing seals are removed therefrom. A plurality of membrane plates 54 are disposed within the frame 53 and are removably fixed thereto. Each membrane plate 54 consists of a membrane support member 57 having passages 55 and 56 at the upper and lower ends thereof, and membranes 58 which are attached to both faces of the membrane support member 57 with a clearance 57a. Numeral 59 denotes outlet ports through which a filtrate is drained. Numeral 60 denotes a sludge inlet, and numeral 61 denotes a sludge outlet. Numeral 62 denotes inter-membrane passages through which sludge or concentrated sludge flows.
Sludge in the circulating tank 51 is led to the sludge inlet 60 of the membrane apparatus 50 by the circulating pump 52. The sludge led to the membrane apparatus 50 flows into the inter-membrane passages 62 directly or via the passage(s) 56, so that the sludge is separated into concentrated sludge and filtrate. The filtrate is taken out to the outside of the membrane apparatus 50 through the outlet ports 59. The concentrated sludge is returned to the circulating tank 51 via the sludge outlet 61 and is mixed with sludge fed from the unillustrated aeration tank. The above-described circulation is repeated.
In the above-described conventional apparatus, sludge is caused to circulate through the inter-membrane passages 62 at a relatively high speed of about 2-2.5 m/s so as to prevent concentration polarization. However, since the circulation amount and pressure losses both increase, the power of the pump must be increased.
When the apparatus is operated while a pressure is applied to the inter-membrane passages, the amount of filtrate temporarily increases. However, this increases the growing speed of gel layers on the surfaces of the membranes, so that the amount of filtrate decreases. To maintain the amount of filtrate large, the power costs increase further.
In the flat membrane type, gel layers can be removed completely by removing unillustrated packing seals, disassembling the frame, and washing the membrane plates. Hence, even when blocking by sludge occurs, the performance of the membranes can be completely recovered. However, since disassembling and checking the membrane apparatus is dirty work, it is desired to reduce the frequency of such work.
Japanese Patent Application Laid-Open (kokai) No. 2-86893 discloses an in-tank flat membrane apparatus in which a flat membrane is immersed into sludge within an aeration tank, and suction is applied to sludge using a suction pump to obtain filtrate. Use of this apparatus decreases the power needed to produce a required amount of filtrate. In addition, by decreasing the amount of filtrate, the growing speed of gel layers can be decreased, and adhesion of sludge can be prevented by bubbles produced by aeration in the aeration tank.
In the in-tank flat membrane apparatus disclosed in that patent publication, since the distance between the air dispersing pipe and the membranes is large, most of bubbles do not enter inter-membrane passages. Therefore, sludge blocking occurs in some inter-membrane passages into which bubbles do not enter. Moreover, when sludge adheres to the membranes, the entire apparatus must be pulled out of the sludge for cleaning. This work is complex and dirty.